1. Field of the Invention
The present invention relates generally to a system for processing a video signal provided from a television camera, and is more particularly directed to method and aparatus for adjusting the white balance of a video signal by use of a simple arrangement including a so-called microcomputer or microprocessor.
2. Description of the Prior Art
In conventional white-balance adjustment systems, such as that described below with reference to FIG. 1, the levels of the red (R-Y) and blue (B-Y) color difference signals provided from a television camera are compared, for instance, in a window comparator, with an upper threshold voltage and with a lower threshold voltage, and an up/down counter is caused to count up and to count down whenever the color difference signal is below the lower threshold voltage and above the upper threshold voltage, respectively. Then, the count on the up/down counter is converted to an analog control voltage and is applied to a gain-controlled amplifier to control its gain. The gain-controlled amplifier, which processes a signal taken from the luminance component provided from the television camera, provides a correction signal which varies with the luminance component and which is proportional to the count on the up/down counter. The correction signal is then combined with the associated color difference signal to provide a white-balance-adjusted color difference signal.
In such a prior art arrangement, if the color difference signal is outside the range defined by the upper and lower threshold voltages, the gain of the gain-controlled amplifier is adjusted so that the level of the associated color difference signal is returned within the range between the threshold voltages.
However, in such a conventional arrangement, the upper and lower threshold values are established by a circuit system apart from the channels which process the color difference signals. As a result, drift error often occurs in the threshold voltages so that, after a lapse of time, it is possible that the conventional white balance adjustment system will be unable to effect accurate correction of white balance.
In addition, because the up/down counters associated with the conventional system count one bit at a time, a significant amount of time is used in making large adjustments in white balance, for instance, in order to accommodate large changes in color temperature of an illuminating light source.